1. Field of the Invention
The present invention relates to the field of data recording and reproduction of a disk drive, and in particular, to a data recording and reproducing technique to which a turbo coding/decoding method is applied.
2. Description of the Related Art
In general, a disk drive, represented by a hard disk drive, is provided with a signal processing circuit (hereinafter referred to as a “read/write channel”) to process a read signal (reproduced data signal) read by a head from a disk as a recording medium or a write signal (record data signal) written to the disk.
The read/write channel employs a so-called PRML (Partial Response Maximum Likelihood) method, a combination of a partial response (PR) method and a Viterbi decoding method.
In recent years, much attention has been paid to a turbo coding/decoding method, which is expected to provide better characteristics than the PRML method. Prior art documents on this method include, for example, “CODING AND ITERATIVE DETECTION FOR MAGNETIC RECORDING CHANNELS”, (Zinning Wu, Kluwer Academic Publishers).
Specifically, a serial concatenated turbo coding and iterative decoding method has been proposed for the read/write channel of the disk drive. This method uses a recursive systematic convolutional (RSC) code to generate a systematic code sequence consisting of an information sequence and a parity sequence. The information sequence corresponds to a recorded data sequence (input information sequence) used for the disk drive. Further, the parity sequence corresponds to an error correction sequence (check bit sequence).
On the other hand, for a decoding system, an APP decoder is used which executes a posteriori probability (APP) decoding based on a soft-output Viterbi algorithm that achieves soft output decoding on the basis of the Viterbi algorithm.
When the concatenated turbo coding/decoding method is applied to the read/write channel, an RSC encoder is used to generate a systematic code sequence consisting of an information sequence and a parity sequence. This systematic code sequence has a coding rate of ½, so that if it is directly written to the disk, processing efficiency may decrease. Thus, the capability of correcting errors may be relatively degraded. However, to increase the coding rate, a decimation process called “puncture” is normally executed on the parity sequence.
However, with the concatenated turbo coding/decoding method, improvement of the error correction capability based on iterative decoding may fail under certain combinations of puncture method and RSC sequence as an outer code. This may result in a failure to obtain a good error rate characteristic (bit error rate characteristic).